Virtually all of the metabolic and developmental effects of thyroid hormone are mediated by 3,3',5-triiodothyronine (T.sub.3), which is produced from thyroxine (T.sub.4) by 5' deiodination. Iodothyronine 5'-deiodination is catalyzed by two general classes of enzymes (Leonard, et al., In: Hennermann, G. (ed.), Thyroid Hormone Metabolism, Marcel Dekker, New York, pp. 289-229 (1986)) distinguished by their tissue distribution, physiological roles, K.sub.m for substrate, and sensitivity to propylthiouracil (PTU).
Type I deiodinase, present predominantly in liver and kidney, provides most of the plasma T.sub.3 in the rat. This class of enzyme exhibits a K.sub.m for T.sub.4 of .about.2 .mu.M and is sensitive to inhibition by PTU. Type II deiodinase, found in pituitary, cerebral cortex, and brown adipose, functions primarily to provide an intracellular source of T.sub.3 for these tissues. This enzyme exhibits a K.sub.m for T.sub.4 of .about.2 nm and is PTU resistant. Many attempts at elucidating the molecular structure of these enzymes are in progress, but these efforts have to date been unsuccessful. Furthermore, the purification of these deiodinases has not been reported. (Berry, et al., Mol. Endocrin. 4:743-748 (1990).)
Regulation of both type I and type II deiodinase activities by thyroid hormone has been examined using tissue homogenates and microsomes (Kaplan, et al., Endocrinology 105:548-554 (1979); Kaplan, M. M., Endocrinology 105:548-554 (1979); Smallridge, et al., Endocrinology 111:2066-2069 (1982)). Activity levels of type I deiodinase in rat liver are low in the hypothyroid state and elevated in hyperthyroidism. Conversely, type II deiodinase activity levels are regulated in the opposite direction,, being low in tissues from hyperthyroid animals and elevated in hypothyroidism (Larsen, et al., Endocr. Rev. 2:87-102 (1981)). Since no specific reagents for quantitation of the enzymes have been developed, it has not been possible to confirm that these activity changes are due to alterations in the enzyme content or, if so, whether they are transcriptional or post-transcriptional.
It has also been suggested that Type I deiodinase, which requires reduced thiols for maximal enzyme activity, is closely related to rat protein disulfide isomerase (PDI) (Boada, et al., Biochem. Biophys. Res. Comm. 155:1297-1304 (1988)). However, this hypothesis has been discredited by at least two other groups, who assert that the clone isolated by Boado et al. actually codes for PDI. (Freeman, R. B., Cell 57:1069-1072 (1989); Schoenmakers, et al., Biochem. Biophys. Res. Commun. 162:857-868 (1989)).
Thus, despite its importance in thyroid hormone activation, Type I iodothyronine 5' deiodinase has not been well-characterized. The need for a DNA sequence encoding Type I iodothyronine 5' deiodinase is clearly recognized in the art.